Prepare a grill for medium heat. Lightly oil the grate.
Divide the ground beef into 6 patties. Season with meat tenderizer, salt and pepper. Set aside.
Melt the butter in a skillet over medium heat. Add the mushrooms and soy sauce; cook and stir until browned. Set aside and keep warm.
Grill the patties for about 6 minutes per side, or until cooked through. Divide the mushroom mixture evenly amongst the burgers and top each one with a slice of Swiss cheese. Cover the grill for about 1 minute to allow the cheese to melt. Remove from the grill and serve on hamburger buns.